Polyvinylidene fluoride (PVDF) is a highly crystalline, chemically resistant engineering thermoplastic fluoropolymer having a typical useful temperature range from −15° C. up to 150° C. Although the glass transition temperature (Tg) for PVDF resins, which normally defines low temperature performance, is near −40° C., in practice, the useful low temperature range for PVDF under impact applications is higher than the Tg. PVDF resins becomes increasingly brittle as its glass transition temperature is approached resulting in poor low-temperature impact resistance. PVDF also loses its flexibility and becomes more rigid at these lower temperatures. It is generally recognized that PVDF resin's low temperature usefulness ranges between +5 and −15° C. Low temperature impact resistance and flexibility are especially important for pipes, hoses and other melt-processed articles used in cold climates.
Low temperature properties of PVDF can be improved by introducing co-monomers during polymer synthesis, to form PVDF copolymers. Copolymers of VDF and perfluoroalkyl vinyl ether (PAVE) have been synthesized as curable elastomers (U.S. Pat. No. 3,136,745) showing good low-temperature performance. Copolymers of vinylidene fluoride (VDF) and hexafluorpropylene (HFP) are shown in U.S. Pat. Nos. 4,076,929 and 4,569,978 to provide better low temperature performance. These copolymers are formed by adding the comonomer either to the initial charge, or at a constant level in a continuous mixed monomer feed to form random copolymers. The problem with these better low-temperature impact PVDF copolymers is that they also have significantly lower melting temperatures than PVDF homopolymers, reducing their useful temperature range.
One method used to optimize PVDF copolymer properties is to make a heterogeneous copolymer by altering the monomer feed during the polymerization, forming an initial polymer that is high in VDF monomer units, generally above 90 weight percent VDF, preferably above 95 weight percent, and in a preferred embodiment a PVDF homopolymer, then adding a co-monomer to the reactor at a point well into the polymerization to produce a copolymer. The VDF-rich polymer and copolymer, if properly selected and in sufficient quantity, will form distinct phases, resulting in an intimate heterogeneous copolymer.
A heterogeneous PVDF copolymer is one having two (or more) distinct phases, with a polyvinylidene fluoride rich phase, and a PVDF copolymer phase that is comonomer-rich. While the co-monomer-rich phase can be of any physical properties, in a preferred embodiment it has elastomeric properties, and will be referred to generally in this application as the “rubber phase”, though the comonomer-rich phase could also be a non-rubber. These phases can form as a non-continuous structure (having discrete rubber domains) or, as surprisingly found in the present invention, as a co-continuous structure. A heterogeneous PVDF copolymer of the prior art will form discontinous, discrete individual rubber phase copolymer domains that are homogeneously distributed in PVDF-rich continuous phase. The co-continuous heterogeneous copolymer of the present invention is one having two (or more) continuous phases that are intimately intertwined with each other and cannot be physically separated.
U.S. Pat. No. 7,863,384 describes a heterogeneous copolymer having a PVDF continuous phase with non-continuos nano-domains of a VDF/HFP copolymer, The nano-domains have a domain size of 20-900 nm, and are refractive index mismatched to the continuous phase, to produce a whiter composition. The comonomer in this case is not added until after at least 90 percent of the VDF monomer has been fed to the reactor.
A heterogeneous copolymer of VDF and PAVE is described in U.S. Pat. No. 7,700,700. Initially only VDF monomer is fed to the reactor until 50 to 90 percent of the total VDF monomer has been fed, and then the total amount of perfluoroalkyl vinyl ether monomer is fed, followed by the remainder of the VDF monomer. The advantage of the heterogeneous polymer formed is that it has the good low-temperature impact properties of the VDF/PAVE copolymer, yet has a melting point much higher than the VDF/PAVE copolymer by itself.
Heterogeneous copolymers of vinylidene fluoride and hexafluoropropylene have been produced by a process in which the comonomer is introduced in the latter stages of the polymerization. (U.S. Pat. Nos. 5,093,427, 6,187,885). US '427 discloses a method in which the comonomer is not added until at least 50-90 percent of the VDF feed has been added, and thus the copolymer makes up a minor amount of the over-all copolymer composition, and exists in distinct “vinylidene fluoride-hexafluoropropylene domains”. In the process of the US'427 patent, 1 to 20 weight percent of hexafluoropropylene is fed to the reactor, of which at most 83% is expected to become part of the copolymer. Although providing improved low temperature impact properties, these heterogenous copolymers tend to be relatively rigid due to the high PVDF homopolymer content which prevents use in applications requiring flexibility at room temperature.
There is a need for PVDF polymers having a combination of a high melting point for good melt processing, improved low temperature properties, and a lower flexural modulus than is found in the art. While other types of fluoropolymers could possibly meet the performance requirements of good low temperature properties and a higher melting point, they are far more costly, and much more difficult to process. There is a need for an alternative to these more costly fluoropolymers, such as fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), ethylene chloro-trifluoroethylene (ECTFE). These needs are met by the composition of the present composition.
Surprisingly it has now been found that heterogeneous PVDF copolymers having high melting points, and improved flexibility and low temperature impact properties can be formed by incorporating the co-monomer into the polymerization at a point between 25 and just under 50 weight percent of the VDF monomer has been added. This finding is surprising based on the disclosure of U.S. Pat. No. 5,093,427 that incorporating the co-monomer into the heterogeneous polymerization before about 50 weight percent of the VDF monomer has been added will depress the melting point. The present invention's earlier addition of the comonomer also allows higher levels of co-monomers such as HFP and PAVE to be incorporated into the copolymer composition, without any significant decrease in the melting point. Higher levels of HFP and PAVE provide better low temperature properties.
Property improvements are in part attributed to higher levels of rubber phase contained in the composition which can improve overall physical properties and surprisingly, also improves rheological characteristics effecting melt processability. The compositions of the present invention optimize the level of the rubber phase to provide unique properties. When the proportion of rubber phase in the total composition is low, the resultant product is semi-rigid, and exhibits less than optimal low temperature impact properties and less desirable rheological characteristics that can adversely affect melt extrusion processes. Conversely, when the proportion of rubber phase is too high, physical properties associated with toughness are adversely effected.
While not being bound by any particular theory, it is believed that the unique co-continuous nature of the copolymer provides a synergy between the separate but interconnected phases, and may have a positive effect on physical properties, including low temperature impact. Further, the co-continuous nature is thought to be responsible for the improved rheological properties. The presence of this co-continuous morphology in a PVDF copolymer is an observable attribute occurring in compositions of this invention.
In summary, by possessing a novel balance between a high VDF polymer phase and VDF copolymer phase, the co-continuous copolymers of the invention provide a unique balance of high and low temperature properties which are not easily attainable by other VDF-based copolymer systems.